Photoconductive varistor

ABSTRACT

A photoconductive polycrystalline varistor comprising a sintered body of zinc oxide and a minor constituent consisting of other metal oxides or halides has a pair of electrodes on one major face thereof. The electrical resistance between the electrodes is a non-linear function of the electrical potential applied between the electrodes and is additionally a function of the intensity of illumination of the interelectrode gap.

O United States Patent 1 111 3,913,055

Levinson Oct. 14, 1975 [54] PHOTOCONDUCTIVE VARISTOR 2,999,240 9/1961 Nicoll 338/ 15 UX 3,011,075 11 1961 T ml 338 20 X [75] Inventor Lionel Schenectady 3,142,586 7/1964 3 3s/15 3,220,881 11/1965 Yando 338/21 X 3 364 388 1/1968 Zin 338/20 X [73] Ass1gnee: General Electric Company,

Schenectady, NY. 3,638,159 l/1972 Kenney 338/20 [22] Filed: July 1, 1974 FOREIGN PATENTS OR APPLICATIONS 831,691 1/1970 Canada 338/20 [21] Appl. No.: 484,480

Related US. Application Data Continuation of Ser. No. 319,330, Dec. 29, 1972, abandoned.

US. Cl. 338/15; 338/20; 338/21 Int. Cl. HOlC 7/08 Field of Search 338/15, 18, 20, 21; 252/501, 518; 29/572; 250/83 R, 83.3 H, 211 R References Cited UNITED STATES PATENTS 10/1956 Thomsen 338/15 X 6/1958 Hester 250/211 R 7/1958 Koury 252/501 X CURRENT Primary ExaminerC. L. Albritton Attorney, Agent, or Firm-Jack E. Haken; Joseph T. Cohen; Jerome C. Squillaro [57] ABSTRACT A photoconductive polycrystalline varistor comprising a sintered body of Zinc oxide and a minor constituent consisting of other metal oxides or halides has a pair of electrodes on one major face thereof. The electrical resistance between the electrodes is a non-linear function of the electrical potential applied between the electrodes and is additionally a function of the intensity of illumination of the interelectrode gap.

10 Claims, 1 Drawing Figure VOLTAGE SENSOR W /-SOURCE U.S. Patent Oct. 14,1975 3,913.055

CURRENT VOLTAGE SENSOR -SOURCE PHOTOCONDUCTIVE VARISTOR This is a continuation, of application Ser. No. 319,330, filed Dec. 29, 1972, now abandoned.

BACKGROUND OF THE INVENTlON 1. Scope This invention relates to photoconductors. More particularly, this invention relates to the discovery and embodiments thereof of a photoconductor effect in polycrystalline varistors comprising sintered bodies of a first metal oxide as a major constituent including additionally an admixture of other metal oxides' and/or halides.

II. Prior Art There are a few known substances whose resistance characteristic is non-linear and is expressed by the equation where I is the current flowing through the material,

V is the voltage applied across the material,

C is a constant which is a function of the physical dimensions of the body, its composition, and the parameters of the process employed to form the body, and

a is a constant for a given range of current and is a measure of the non-linearity of the resistance char acteristic of the body.

A well-known example of such varistor materials is silicon carbide. Silicon carbide and other non-metallic varistor materials are characterized by having an alpha exponent of less than 6. Recently, a family of polycrystalline metallic oxide varistor materials have been produced which exhibit an alpha exponent in excess of 10. These new varistor materials comprise a sintered body of zinc oxide crystal grains, including additionally an intergranular layer of other metal oxides and/or halides, as, for example, beryllium oxide, bismuth oxide, bismuth fluoride, or cobalt fluoride, and are described, for example, in US. Pat. No. 3,682,841, issued to Matsuoka et al on Aug. 8, 1972 and US. Pat. No. 3,687,871, issued to Masuyama et al on Aug. 29, 1972.

Substantial effort has been expended in research into the charateristics and applications of this new varistor material, and has resulted in the making of a number of inventions, as for example, those described in US. Pat. Nos. 3,693,053 and 3,694,626 which issued respectively to Thomas E. Anderson on Sept. 19, 1972 and John D. Harnden, Jr. on Sept. 26, 1972 and which are assigned to the 'assignee of this invention. Until now, however, attempts to produce a photoconductive polycrystalline metal oxide varistor have been unsuccessful.

Photoconductivity of zinc oxide itself is known in the art. Photoconductivity of pure polycrystalline and monocrystalline zinc oxide was experimentally measured and reported by Harrison in 93 Physical Review No. 1 at page 52 in 1954. Furthermore, photoconductivity of zinc oxide and other Group II-Group Vi compounds, as for another example, cadmium sulfide, is a theoretically predictable phenomenon.

The photoconductivity of zinc oxide, however, does not contribute photoconductivity to zinc oxide based polycrystalline varistors because the zinc oxide grains are relatively highly conductive and so any radiation induced change in resistivity of the zinc oxide grains which may occur is not observable because of the high resistance of the intergranular material comprising oxides and/or halides of other metals. Accordingly, any photoconductivity to be found in these polycrystalline varistors must result from photoconductivity of the intergranular material. Heretofore, such photoconductivity has not been observed despite experimental efforts to obtain the effect.

111. Cross-Reference to Related Copending Application This invention is related to the copending application of Levinson and Philipp, now US Pat. No. 3,862,422 filed of even date herewith, assigned to the assignee of this invention, and incorporated herein by reference thereto.

Prior work with polycrystalline metal oxide varistor devices has been directed to devices providing current conduction through the bulk thereof. 1 have discovered that photoconductivity cannot be observed in such devices because of insufficient penetration of energy into the device.

Accordingly, it is an object of this invention to provide a photoconductive polycrystalline varistor.

Another object is to provide such a device which has simultaneous utility as a photodetector and as a varistor.

Briefly, and in accordance with one embodiment of this invention, there is provided a photoconductive polycrystalline metal oxide varistor in the form ofa sintered pellet or disk having a pair of electrodes disposed on one surface thereof. The electrodes have a gap therebetween which provides for the conduction of electrical currents from one electrode to another through a portion of the varistor material and provides for illumination of the current-carrying portion of the varistor material.

The novel features of this invention sought to be patented are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may be understood from a reading of the following specification and appended claims in view of the accompanying drawing in which the single FIGURE is a plan view of a photoconductive varistor in accordance with this invention.

The FIGURE shows a body of polycrystalline varistor material 10 having a surface on which are disposed first and second electrodes 11 and 12. Electrodes l1 and 12 have respectively substantially parallel facing edges 13 and 14 which cooperatively define a gap 15 between electrodes 11 and 12 on the surface of polycrystalline varistor 10.

Varistor 10 is shown in the drawing as having a circular shaped surface and represents a disk or plinth of polycrystalline varistor material. The disk or plinth shape is a convenient one to manufacture, but this invention is not so limited and other shapes, as for example, a rectangular prism shape may be employed if desired for design reasons.

Polycrystalline varistor disk 10 is fabricated by pressing and sintering a mixture comprising zinc oxide as a major constituent and other metal oxides and/or halides as minor constituents. For a more detailed general description of the composition and method of fabrication of the polycrystalline varistor disk, reference is made to the aforementioned patents of Matsuoka et al, Masuyama et al, Anderson, and Harnden, Jr. in one embodiment of this invention the varistor disk is comprised by weight of principally zinc oxide with bismuth oxide and antimony oxide as secondary constituents and smaller amounts of each of other metal oxides. This embodiment exhibited a varistor alpha exponent of 40.

Electrodes l1 and 12 are applied to a base of polycrystalline varistor disk in a convenient manner known in the art and may, for example, be applied as silver paint or as evaporated or sputtered metals such as aluminum, zinc, or platinum. The spacing between parallel facing edges 13 and 14 of the electrodes is selected to determine the varistor voltage of the device. The varistor voltage is accordingly a function of the width of gap 15. Gap 15 also provides for illumination of the current-carrying portion of varistor disk 10.

In operation, the application of an electrical potential difference between electrodes 11 and 12 causes a current to flow between the electrodes 11 and 12 through portion 15 of varistor disk 10. The magnitude of this current is given by the equation:

V a I B T where B is a positive function of the intensity and frequency of electromagnetic energy illuminating gap 15, and of the electrical potential across gap 15; the other parameters being as identified above.

The electrical potential may be applied between the electrodes 11 and 12 by means of a series connected voltage source 17 which may be any of the many types known to the electrical arts; for example, an electrochemical cell or a line powered voltage source circuit. The magnitude of the electrical current flowing through the varistor disk 10 between the electrodes 11 and 12 may be measured by means of a current sensor 16 connected in series with the voltage source 17 and the electrodes 11 and 12. The current sensor 16 may be of any type, for example, an electromagnetic milliammeter or oscilloscope current probe. The devices for applying electrical potential and for sensing electrical current flow are well known to the art and the abovementioned devices are typical but not inclusive of the means which may be employed in the practice of this invention.

There is thus provided a unitary device which provides simultaneously and in combination the functions of a varistor and a photoconductor. While this invention has been described with reference to a particular embodiment, other modifications and variations will occur to those skilled in the art in view of the above teachings. Accordingly, it should be understood that within the scope of the appended claims the invention may be practiced otherwise than is specifically described.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A photoconductive varistor comprising:

a body of sintered polycrystalline, varistor material comprising zinc oxide and bismuth oxide and further characterized as comprising a plurality of zinc oxide grains of relatively high conductivity and an intergranular layer of material of relatively low conductivity and having at least one surface; and

a pair of electrodes disposed on said surface and having a gap therebetween, said gap providing for illumination of said surface by electromagnetic energy and providing an electric current path between said electrodes through said polycrystalline varistor material.

2. The photoconductive varistor of claim 1 wherein said polycrystalline varistor material comprises a mixture of metal oxides and wherein the magnitude of an electric current flowing in said electric current path is given by the equations:

V a 1 C when said gap is not illuminated by electromagnetic energy, and

' I=B T when said gap is illuminated by electromagnetic energy.

3. The photoconductive varistor of claim 2 wherein said polycrystalline varistor material comprises a mixture of metal oxides and metal halides.

4. The photoconductive varistor of claim 2 wherein said metal oxides include zinc oxide as a major constituent and bismuth oxide and antimony oxide as minor constituents.

5. The photoconductive varistor of claim 1 wherein said electrodes have substantially parallel facing edges, said gap being between said edges.

6. The photoconductive varistor of claim 1 wherein said electrodes are vapor deposited metal electrodes.

7. The photoconductive varistor of claim 1 wherein said electrodes are silver paint electrodes.

8. A method of using a sintered body of polycrystalline metal oxide varistor material including zinc oxide as a major constituent and minor constituents selected from the group consisting of other metal oxides and halides, said method providing for photodetection and comprising the steps of:

applying a pair of electrodes to a surface of said body of polycrystalline metal oxide varistor material including zinc oxide as a major constituent and minor constituents selected from the group consisting of other metal oxides and halides;

applying an electrical potential between said electrodes;

positioning said surface to receive electromagnetic radiation energy; and

measuring an electrical current flowing between said electrodes in response to said electromagnetic energy.

9. The method of claim 8 wherein the magnitude of said electrical current is given by the equations when said surface is not illuminated by electromagnetic energy, and

tially parallel facing edges on said surface. 

1. A PHOTOCONDUCTIVE VARISTOR COMPRISING: A BODY OF SINTERED POLYCRYSTALLINE, VARISTOR MATERIAL COMPRISING ZINC OXIDE AND BISMUTH OXIDE AND FURTHER CHARACTERIZED AS COMPRISING A PLURALITY OF ZINC OXIDE GRAINS OF RELATIVEY HIGH CONDUCTIVITY AND AN INTERGRANULAR LAYER OF MATERIAL OF RELATIVELY LOW CONDUCTIIVITY AND HAVING AT LEAST ONE SURFACE: AND A PAIR OF ELECTRODES DISPOSED ON SAID SURFACE AND HAVING A GAP THEREBETWEEN, SAID GAP PROVIDING FOR ILLUMINATION OF SAID SURFACE BY ELECTROMAGNETIC ENERGY AND PROVIDING AN
 2. The photoconductive varistor of claim 1 wherein said polycrystalline varistor material comprises a mixture of metal oxides and wherein the magnitude of an electric current flowing in said electric current path is given by the equations:
 3. The photoconductive varistor of claim 2 wherein said polycrystalline varistor material comprises a mixture of metal oxides and metal halides.
 4. The photoconductive varistor of claim 2 wherein said metal oxides include zinc oXide as a major constituent and bismuth oxide and antimony oxide as minor constituents.
 5. The photoconductive varistor of claim 1 wherein said electrodes have substantially parallel facing edges, said gap being between said edges.
 6. The photoconductive varistor of claim 1 wherein said electrodes are vapor deposited metal electrodes.
 7. The photoconductive varistor of claim 1 wherein said electrodes are silver paint electrodes.
 8. A method of using a sintered body of polycrystalline metal oxide varistor material including zinc oxide as a major constituent and minor constituents selected from the group consisting of other metal oxides and halides, said method providing for photodetection and comprising the steps of: applying a pair of electrodes to a surface of said body of polycrystalline metal oxide varistor material including zinc oxide as a major constituent and minor constituents selected from the group consisting of other metal oxides and halides; applying an electrical potential between said electrodes; positioning said surface to receive electromagnetic radiation energy; and measuring an electrical current flowing between said electrodes in response to said electromagnetic energy.
 9. The method of claim 8 wherein the magnitude of said electrical current is given by the equations
 10. The method of claim 8 wherein said step of applying a pair of electrodes more particularly comprises vapor depositing a pair of electrodes having substantially parallel facing edges on said surface. 